Oil well casing



Sept. 16, 1941. T. J. CRAWFORD OIL WELL CAS ING Filed March 30, 1940INVENT OR. 77/0/7/4 S I CPA WF'FD BY y WW ATTORNEYS Patented slept. 16,1941 OIL WELL CASING Thomas J. Crawford, Youngstown, Ohio, assignor toRepublic Steel Corporation, Cleveland, Ohio, a corporation oi' NewJersey Application March 30, 1940, Serial No. 327,018

3 Claims.

This invention relates to the art of steel pipeand its manufacture andis particularly concerned with steel casing for use in deep oil wells,and to a new and improved method for making oil well casing which willhave improved resistance to collapse and improved joint strength.

Steel casing for oil wells is subjected to external forces whichincrease with the depth of the well and which must be resisted to avoidfailure of the casing. Such casing is made in pipe lengths which areabout 40 feet long and are connected together by couplings threaded tothe ends of adjacent lengths of pipe. Since the weight of the string ofcasing is carried by these threads and couplings, the threaded portionsof the pipes must have a high joint strength to prevent parting of thestring and to permit removal of the string from the well should theoccasion demand. Joints having standard threads usually part because ofthe radial compression of the threaded ends of the pipe which takesplace under tension, such distortion allowing the threads to disengage.Joints having acme or square threads usually fail by breaking on thepipe at the thread root beyond the last engaged thread. Furthermore, oilwell casing must be sulciently ductile to resist breakage in instanceswhere the drilled hole is not perfectly straight.

One important specification for oil well casing has been that ofrelatively high ductility. Such high ductility has been accompanied bycollapse strengths and joint strengths which were not as high as weredesirable. Various expedients have been tried to increase the collapseefliciency and joint strengths of casing without materially decreasingthe ductility of the casing, but so far as I know these efforts have notbeen entirely satisfactory.

The American Petroleum Institute, appreciating the need in the industryfor a joint of higher strength, yet using standard thread dimensions andcouplings, has recently appointed an engineering committee to work onthe problem of designing such joints on American Petroleum Institutestandard 51/2 and 7 casing where rotary drill pipe is not used insidethe casing except in emergencies. One proposal is to upset the threadedends of the pipe internally to provide suilcient metal under the threadsto give the desired strength.

By the present invention I am able to increase joint strengths of casingnot Aonly of those sizes but also of larger sizes within which drillpipe is used for drilling and accomplish these results without the extracost, reduced clearances and localized wear which are inherent factorsin upset pipe ends.

By the present invention I am able to increase the collapsestrength ofcasing as much as 25% or more and to increase the joint strength as muchas 50% or' more, without materially decreasing the ductility of thecasing. Briey stated, I increase the collapse strength and eiliciency ofthe casing in any one of several ways. I may quickly heat a thininterior layer of the casing to a temperature between about 500 F. andabout 1000 F. and allow the pipe to cool; or I may quickly heat a thininterior layer to above the Aca temperature of the steel and quench itand, if and when necessary to improve ductility. I may temper or drawthe' resultingvhardened layer by quickly reheating it to a temperaturebetween about 500 F. and about 1000" F. and allowing the pipe to cool.Where it is desired to preserve the effects of previous cold working ofthe pipe in the portions thereof which have not been hardened by heattreatment, the drawing operation should be carried out rapidly andlocally so that the heating eifect is confined to the hardened material.When the drawing operation undesirably changes stresses in the pipe andit is desirable -to improve the stresses, I may quickly and locally heatthe exterior of the pipe to a temperature between about 500 F. and about1000 F. and allow it to cool.

I increase the joint strength by quickly heating a -thin interior layerof the casing at and adjacent to the pipe ends to a temperature abovethe Aca temperature of the steel and then quenching and hardening thisheated layer.

In the drawing accompanying and forming a part of this speciilcation,

Figure 1 is a perspective view, partly in section, of a length of oilwell casing embodying the present invention; and

Figure 2 shows curves illustrating collapse emciencies of casing and theeffects thereon of the present invention.

In Fig. 1, i designates a length of oil well casing, 2 designates theexterior threads at each end thereof which are to engage with couplings(not shown) and 3 indicates the thin hardened layer of metal on theinside of the casing within the threaded portions. In general, the ratioof the diameterI of the casing to the wall thickness, i. e., D/t, rangesfrom about 12 upwardly, and the present invention insofar as improvingcollapse strength is concerned is directed to steel casing having a D/tratio of more than about 14.

One convenient and satisfactory method of treating casing such as isshown at I in Fig. 1 to improve its collapse eiciency, is as follows: Anelectric induction heating element in the form of a short cylindricalmember slightly less in diameter than the casing to be treated isinserted in the casing and is connected to a source of electrical powerwhich is sumcient to heat the inner surface of the pipe by induction toa temperature between about 500 F. and about 1000 F., within a veryshort time, for example within a few seconds. Relative axial movement ofthe heater and pipe is created, and preferably the heater alone ismoved. 'Ihe rate of this relative movement may range from about 1%"upwardly to 15" or more per second, depending upon the power inputavailable. It will be understood that the speed of such relativemovement may be varied depending on the power input, but the speed andpower input should be' so regulated that only the inner surface or avery thin inner surface layer is heated to the desired temperature, andthat this heating is done very rapidly and, in effect, locally for it isimportant that the casing 'should not be heated thruout its wallthickness to 'th'e peak temperature but that the heat supplied to thethin inner layer should dissipate itself by spreading to the remainderof the casing wall.' Preferably the pipe is rotated during relativeaxial movement of the pipe and heater to overcome the eiects ofvariations in clearance therebetween.

When a plurality of lengths of steel casing havlng a D/t ratio of morethan 14 is so treated, the average collapse efciency will be improvedand the statistical minimum values on which design factors of safety arebased will be raised still vmore in proportion.

I have found that collapse strength in steel casing having a D/ t ratioof 14 or more is affected by the presence of stresses which, for lack ofa better description, have been called tangential residual stresses. Ifa short length or ring of casing having such stresses is cutlongitudinally, i. e., axially, the casing tends to spring open, therebyopening the cut, when the tangential residual stresses are positive, andtends to spring inwardly, thereby closing the cut, when these stressesare negative. I have also found that when a length of casing ofhomogeneous structure possesses substantially 'no` tangential radialstresses its collapse strength and eiliciency isat the maximum, and thatas these stresses increase in magnitude, whether they are positive ornegative, the collapse strength and efficiency decreases.

Steel casing having a D/t ratio above 14 as ordinarily made heretofore,usually-as a result of cold rotary straightening-possessed tangentialresidual stresses which were positive and which varied in amounts from afew thousand pounds per square inch to 20,000 pounds per square inch ormore. These stresses can be diminished and in some cases reversed frompositive to negative values by the treatment above described. As thesestresses approach a minimum, the collapse strength and efficiencyincrease.

Curve 5 of Fig. 2 illustrates not only the relationship existing betweencollapse eiciency and transverse residual stresses in steel casinghaving a D/t ratio of 14 or above, but also the improvements in collapseefficiency which I have obtained by means of the above describedtreatment. Lengths of casing which have been straightened while cold `bythe rotary method have positive tangential residual stresses of variousamounts usually greater than about 5000 lbs. per square inch and areillustrated by the upper part of curve 5. When a plurality of suchcasing lengths is treated as above described, those stresses aredecreased and may be reversed with the result that the group of pointsrepresenting the stresses in the various casings is moved closer to thenode of the curve and the average and the minimum collapse efficienciesare increased. As curve 5 indicates, the collapse eillciency of a lengthof such casing is about when substantially no transverse residualstresses are present.

The point A on curve 5 indicates a similar length of casing which,beforebeing treated as above described, had a positive transverseresidual stress of about '7000 pounds per square inch and a collapseefficiency of about 95%. Point B illustrates a length of casing whichhad a positive tangential residual stress of about 9400 pounds persquare inch and a collapse eiiiciency.

' straightened casing.

The illustrative figures were obtained on homogenous casing, i. e., apipe which is substantially uniform in structure thruout its wallsection. These results were obtained with steel containing between about.20% and about .24% of carbon and between about .61% and about .65% ofmanganese.

Instead of using the low temperature treatment described above, I mayuse a higher temperature and harden an inner thin layer of the casing.As briefly described above, this treatment consists of quickly heating athin inner layer of the casing to above the Aca temperature of the steeland then quenching and hardening that layer.

Curve 6 of Fig. 2 illustrates improvements in collapse strength andeiciency which have been obtained by such a high temperature treatment.The specimen illustrated by E on curve 6 had a positive tangentialresidual stress of about 22,000 pounds per square inch with anaccompanying collapse efciency of about 125%. Similarly, anotherspecimen illustrated by D had a positive transverse residual stress ofabout 15,000 pounds per square inch and a collapse eiciency of about127%. F represents another specimen whose positive transverse residualstress was about 9400 pounds per square inch and whose collapseeiiciency was about When casing is provided with such a hardened innerlayer its ductility is decreased somewhat. 'I'he ductility may beimproved by reheating the hardened layer quickly and, in effect, locallyas previously described, i. e., by heating it rapidly to between about500 F. and about 1,000 F. and allowing it to cool. Point G on curve 6illustrates casing similar to the casing of points D and E but drawn asjust described. This specimen had a collapse efciency of about 88%, anegative tangential stress of about 28,320 lbs. per square inch andincreased ductility (not shown by the curve) as compared with itsductility before this drawing treatment.

The collapse efliciency of casing such as is illustrated by the hardenedand drawn specimen of point G on curve 6 may be increased by subjectingthe exterior surface to a low temperature treatment, for example, byquickly heating the outer surface to a temperature between about 500 F.and about 1000 F. and allowing it to cool. Such a treatment willdecrease the negative transverse residual stress and increase thecollapse efliciency.

'Ihe joint strength may be increased in the following manner: A heatingelement as above described is inserted within the end of the pipe to bethreaded and suflicient electrical power is employed to heat the in nersurface layer of the casing to a depth of, for example, .075" to atemperature above the Aca point of the steel, for example, to betweenabout 1600 F. and about 1800 F. for steel of the foregoing composition,and to do this heating within a very short period of time, for exampleonly a few seconds. The heater may remain stationary if it is ofsufficient length to heat the desired length of the casing at andadjacent to the part to be threaded, or, if the heater is not that longthe heater and casing may be moved relatively at a speed which may rangeupwardly to about 15" per second or more, depending upon the powerinput. Immediately after the surface of the casing has been heated tothe desired temperature it is quenched, as by spraying water on it. Thisquenching may be accomplished by first removing the heater or by movingthe heater in the pipe and following it with a quenching spray,

The effect of this treatment is to increase the hardness of the highlyheated and quenched inner layer. For example, I have obtained innerhardened layers having hardnesses of from about 30 to about 46 on theRockwell C scale, while the hardness of the remainderpf the pipe wallwas about 90 on the Rockwell B scale. The untreated pipes failed inpullout tests under loads of about 175,000 pounds, while correspondingpipes treated as just described failed at pullout loads averaging about315,000 pounds. Thus pipes so treated had strengths which were increasedby about 80%.

The heating and quenching treatment somewhat reduced the ductility ofthe pipes at their ends but this decrease was not serious because of thesupport afforded by the coupling. However, the ductility of pipes thusheat treated and quenched at their ends may be increased by a drawingtreatment, i. e., by rapidly heating the thin hardened layers to betweenabout 500 F. and about 1000 F. In other words, the first collapseimproving treatment described above may be carried out on the heattreated and hardened pipe ends with the result that the ductility willbe considerably increased without much decrease in joint strength. Forexample, pipes having heat treated and quenched ends as above notedfailed in a pullout test at about 315,000 pounds, which was about 180%of the strength of untreated pipes. After being treated by this drawingoperation the pullout failure strength dropped to about 287,000 pounds,which was still about 155% of the corresponding strength in untreatedpipes.

When a length of casing is subjected to the high temperature andhardening treatment the improvement in joint strength will be obtainedsimultaneously with the improved collapse elllciency. `When the hardenedlayer is to be drawn to improve ductility, the drawing treatment ispreferably not applied to the end portions which are to be threaded.

From what has been said hereinabove, it will be understood that eitherthe collapse improvlng treatment or the joint strength improvingtreatment may be carried out together or independently of one another.For example, if it is desired to increase both the collapse and jointstrengths of a 'length of casing, the heater may be passed axially thruthe length of pipe and the heat may be so adjusted as to raise thetemperature at one end to be threaded to above the Ac: point and thendecreased to heat the portion of the pipe between the ends to bethreaded to a temperature in between about 500 F. and about 1000 F. andthen the heat may be increased when the heater comes to the other end tobe threaded so that it will be heated to the temperature used at the rstend. When this method is employed, the highly heated inner surface ofeach end is quenched immediately after the heating. This mayconveniently be done by embodying in the heater a water-carrying tubethru which water may be sprayed onto the heated walls immediately backof the moving heater. When this procedure is followed and it isdesirable to increase the ductility of the hardened end portions of thepipe, the heater may be reemployed to reheat the quenched portions ofthe pipe to a drawing temperature, i. e., from about 500 F. to about1000 F.

It will be understood that it is not essential to use an electricinduction heater as described above but that any other heating means maybe employed which is suitable for carrying out the present invention.

The subject-matter described but not claimed in this application isbeing claimed in copending application Ser. No. 407,350 filed August 18,1941.

Having thus described my invention so that others skilled in the ari;may be able to practice the same, I state that what I desire to secureby Letters Patent is defined in what is claimed.

What is claimed is:

1. In a thin walled steel pipe which is exteriorly threaded at its endsand which is to be 'subjected to heavy tensile stresses in use, meansfor increasing, in the region of the threaded ends, the normalresistance of the pipe to tensile failure and to radia1 deformationresulting in failure by thread disengagement of the threaded joint, saidmeans comprising a thin integral layer of steel at the inner surface ofeach threaded end, said layers being harder than the thread carryingportions of the pipe and being substantially coextensive with saidthreads longitudinally of the pipe.

2. In a thin walled steel pipe which is exteriorly threaded at its endsand which is to be subjected to heavy tensile stresses in use, means forincreasing, in the region of the threaded ends, the normal resistance ofthe pipe to tensile failure and to radial deformation resulting infailure by thread disengagement of the threaded joint, said meanscomprising a thin integral layer of steel at the inner surface oi' eachthreaded end, said layers being harder than the thread carrying metal,being spaced apart radially from the roots of the threads and beingsubstantially coextensive with the threads longitudinally oi' the pipe.

3. In steel pipe having a D/t ratio of above about 14 and havingexteriorly threaded ends, for use as' casing in oil wells, means forincreasing the normal joint strength of said pipe comprising thinintegral inner surface layers within and substantially coextensive withsaid threaded ends longitudinally of the pipe, said inner layers havinga hardness of above about 30 on the Rockwell C scale and being harderthan ,the remainder of the steel in said threaded ends.

THOMAS J. CRAWFORD.

DISCLAIMER 2,256,455.4-Tlwmas J. Crawford, Youngstown, Ohio. OIL WELLCAsINe. Patent threade dated September 16, 1941. Disclaimer led March 1,1943, by the assignee, Republic Steel O'orporalz'on.

Hereby enters this disclaimer, to Wit: that the term substantiallyco-extensive which apearsin claims 1, 2, and 3 shall mean that thehardened layers and the portions are substantially equal in length andthat the hardened layers are not substantially longer than the threadedportlons.

[Qic'al Gazette March 80, 1943.]

